Several methods have been successfully used for controlling the orientation of liquid crystal to balance the direction of liquid crystal molecules and to extend its viewing angle characteristics. Obtaining a wider viewing angle is particularly important to a liquid crystal panel with a thin film transistor. To control the orientation of a liquid crystal, a selected portion of an organic polymer, such as polyimide, is coated on the liquid crystal cell that corresponds to a single pixel. It is then selectively exposed by a deep ultraviolet laser, having a wavelength in the range of 200 to 300 nm.
To selectively expose a predetermined portion of a liquid crystal cell, it is necessary that the exposure be patterned. By way of example, in a 10-inch VGA color display, an exposure range of about 200 mm.times.150 mm that includes a set of 480.times.640 (.times. three colors) rectangular liquid crystal cells, a portion of each liquid crystal cell, e.g., a range about 100 to 150 .mu.m in height along the upper and lower boundaries, must be selectively exposed.
Methods for applying a desired intensity of light at a selected location of such an object on a plane include:
(1) exposure methods that use a mask, such as direct exposure and projection exposure; and PA1 (2) beam exposure methods that use a raster or a vector scan device. PA1 (a) it is difficult to irradiate the overall object with light of uniform intensity; PA1 (b) for projection exposure, aberrations must be minimized over the entire object, which necessitates an advanced optical system; PA1 (c) if a high coherent light source, such as a deep ultraviolet laser is used, interference fringes are likely to occur, thereby preventing a uniform exposure; PA1 (d) since light on the portion that is not exposed is blocked by a mask, the usage of light is low if the ratio of the transmissive portion of the pattern to be drawn is small. PA1 (e) if a wide range is exposed as in a liquid panel, a large lens is required. PA1 (f) The optical materials available for use in obtaining a transmission factor that is required for light of short wavelength, such as deep ultraviolet rays, are limited to synthetic liquid crystal and fluorite. These materials are expensive and difficult to process; furthermore they cannot be formed as aspheric lenses. It is thus difficult to design a combination of lenses for minimizing aberration. PA1 (g) If an optical material other than the above described materials is used, e.g., glass, the necessary transmission factor cannot be obtained. (h) It is likewise difficult to obtain a high optical output, since the use of light over a substantial application time must be increased in addition to the transmission factor. PA1 (i) Saw-tooth waves are used as drive currents for controlling the horizontal deflection. Only the outgoing route for spot movement is used for exposure, while the incoming route, i.e., the retrace line, is not used to move the spot as rapidly as possible. PA1 (j) Since light is blocked when using an optical shutter while the portion that does not require exposing is being scanned, the use of light is limited. PA1 (k) If a wide range is exposed, the difference between the distance between a light source and the center of the object and the distance between the source and the periphery of the object may cause pincushion aberration, wherein the horizontal track of the spot is not a straight line but a curve bending toward the center.
The exposure method that uses a mask as in (1) is known to suffer from the following limitations:
In the beam exposure method in (2), a deflecting device routinely includes a polygon mirror or a galvanomirror. When scanning with a polygon mirror, a beam deflected by the mirror moves in a predetermined direction at a specific angular velocity. An f.THETA. lens commonly used in laser beam printers is focused on the object to maintain the scanning velocity of the spot on the object over a wide range of the deflection angle .THETA. from its center. This method suffers from the following drawbacks:
When scanning with a galvanomirror, a deflecting device can be placed between the lens and the object, since the angular velocity of the mirror can be controlled. This allows to regulate the angular velocity of the mirror to maintain constant the velocity of the spot on the object, without having to resort to f.THETA. lens. If the position of the focal point must vary between the middle and the periphery of the object, an actuator for moving the lens on its optical axis to adjust its focal point can be installed to solve the problem. If, on the other hand, raster scan is used, the following problems occur: